Optical fibers are rapidly becoming ubiquitous as a communication medium. One reason for the growing popularity of optical fibers is advancement in the ability to multiplex many wavelength channels onto a single fiber, and particularly onto a single-mode fiber, which has the advantageous property of relatively low dispersion. For example, dense Wavelength Division Multiplexer (WDM) equipment is now commercially available for multiplexing eighty or more wavelength channels onto a fiber.
WDM technology, together with design of low-dispersion fiber and of fast optical sources and detectors, has advanced so far that 10 Gibabits per second are now readily transmitted over a single fiber using commercial technology, and hundreds, or even thousands, of Gigabits per second can be transmitted using experimental technology.
With the rapid rise in transmission capacity, there has come a concomitant need for protection of sensitive information being transmitted. Although encryption algorithms are available, there are certain drawbacks to their use. For example, they occupy extra bandwidth to a generally inflexible degree, they demand the use of data-processing equipment at the sending and receiving ends for encryption and decryption, respectively, they may involve complicated protocols, and key exchange may be attended by stringent concerns for security.
Thus, it would be useful to provide an alternative to conventional methods of encryption. In particular, it would be useful to have a data-protection technique that is inherently secure against interception of signals, irrespective of whether the transmitted messages are encrypted.